This invention relates to techniques for low cost scrambling and descrambling of audio information signals. More particularly, this invention relates to a lower cost Hi Fi descrambler with an improved performance over the prior art.
The prior art in the art of audio scrambling and descrambling utilized various frequency shifting techniques. The prior arts in audio descrambling suffer from hiss in the form of "white noise", and more importantly in band carrier "whistle caused by intermodulation of the two carrier frequencies. The prior arts also use expensive circuitry such as band pass filters for mixer circuits, wide band 0 degree and 90 degree all pass networks and 0 degree and 90 degree circuits for varying the carrier frequencies with constant amplitude and the need for adjustments to balance gain of quadrature mixers for sideband elimination. In addition, since the mixers used in the prior art are generally not stable in time, their drift results in an audible whistle as the result of carrier leak through.
The prior art requires mixers that require a pure sine wave modulation, therefore a truly analog multiplier is needed. Truly analog multipliers tend to have noise problems because of their circuit configuration that cause white thermal or shot noise components that degrade the signal to noise (SNR) of the audio scrambling system.
Prior art systems having one or more of the identified problems include U.S. Pat. Nos. 4,636,853 ('853), DYNAMIC AUDIO SCRAMBLING SYSTEM, by Forbes issued on Jan. 13, 1987, 5,058,159, METHOD AND SYSTEM FOR SCRAMBLING AND DESCRAMBLING AUDIO INFORMATION SIGNALS by Quan issued on Oct. 15, 1991, and 35,159,631, AUDIO SCRAMBLING SYSTEM USING IN BAND CARRIER, by Quan et al. issued on Oct. 27, 1992 ('159).
A review of the prior art for a full understanding of the present invention will be helpful. Turning now to the drawings, FIG. 1 is a block diagram of the key elements of the Forbes '853 prior art. The Forbes '853 descrambler 10 has a scrambled audio input 34 which is connected to an all pass phase shifter 20 containing a 0 deg. output 38 and a 90 deg. output 39. The scrambled audio signal has an offset frequency 36 f.sub.1 -f.sub.2 as shown in FIG. 2a. This shows the scrambled audio offset by an offset frequency determined by the scrambling process. The phase shifted outputs are connected to a first input of linear modulators 21 and 27.
A frequency generator 22 generates a square wave frequency (f.sub.1) which is fed to band pass filter 24 to remove any harmonics, thus producing a pure sine wave. This f.sub.1 sine wave is connected to a 0 deg. and 90 deg. phase shifter 25. The outputs of phase shifter 25 are in turn connected to second inputs of linear modulators 21 and 27 respectively. The outputs of the first and second linear modulators are added in summer 28 to produce signal 37. This output signal 37 is connected to a first input of a second mixer 30 via high pass filter 29 which passes only f.sub.1 and the upper sideband as shown in FIG. 2b.
A second square wave frequency generator 23 generates a signal f.sub.2 as shown FIGS. 1 and 2c. This square wave is filtered by band pass filter 26 to remove any harmonics to produce a pure sine wave signal. This pure sine wave signal is connected to a second input of third mixer 30. The output of the third mixer 30 is connected to a low pass filter 31 to produce a descrambled output signal 35, as shown in FIG. 2d and 30 as shown in FIG. 1.
The second spectral diagram in FIG. 2b shows the input to the 3rd mixer 30. The frequency f.sub.1 here represents the residual carrier feed through from mixers 21 and 27. FIG. 2c shows a shows the relationship of a carrier f.sub.2 to f.sub.1 in FIG. 2b and the scrambled audio signal shown in FIG. 2a. FIG. 2d, shows the relationship of the spectral characteristics of the descrambled signal 35 and the residual difference frequency (f.sub.1 -f.sub.2) component to the spectral characteristics of the signals in FIGS. 2a-2c.
FIG. 4 shows the scrambled audio input of the Quan prior art descrambler 11. This shows the scrambled audio 40 offset by an offset frequency determined by the original scrambling process. As shown in FIG. 3, the scrambled audio input signal 40 is connected to an all pass shifter 4 which provides 0 deg. and 90 deg. phase shifted outputs 42 and 43 to first inputs of first and second mixers 44 and 45.
Carrier frequency generator 46 generates a sine wave signal f.sub.c 47 with a frequency of 1 Khz or 2-3 khz. The carrier frequency 47 is filtered by a low pass filter 48 to remove any harmonics to produce a pure sine wave 49. This pure sine wave signal 49 is connected to an all pass phase shifter 50 to produce 0 deg. and 90 deg signals 51 and 52 which in turn are connected to second inputs of mixers 44 and 45. The outputs of mixers 44 and 45, signals 53 and 54 are connected to summer 55 to produce descrambled output 56.
FIG. 4b shows the relationship of the in band descrambling carrier f.sub.c to the scrambled audio signal. FIG. 4c shows the descrambled audio spectrum with the residual carrier f.sub.c that is typically -60 db below the descambled audio program, but is still audible during silent passages of the audio program.
It is therefore an object of this invention to provide a higher performance descrambler and/or lower cost frequency shifted scrambled audio signals. The method and apparatus described 1) eliminates the use of 0 degree and 90 degree phase shift circuits, 2) eliminates the use of quadrature mixer circuits, 3) eliminates the need for band pass filters or low pass filters for the modulating carrier, 4) reduces white noise and cost using switching type mixer circuits instead of linear mixers, 5) eliminates in-band audible whistle via filtering out the residual first carrier whistle; 6) eliminates the need to adjust mixers for minimum in-band carrier whistle and 7) since the SNR has been improved the need for noise reduction circuits has been eliminated.